Animal Intelligence Part 8

From the disk in various directions there radiate ant-roads or avenues, which are cleared and smoothed like the disk itself, and which course through the thick surrounding gra.s.s, branching and narrowing as they go till they eventually taper away. These roads are usually three or four in number before they begin to branch, but may be as many as seven.

They are usually two to three inches wide at their origin, but in large nests may be as much as five. MacCook found no road longer than sixty feet, but Lincec.u.m describes one of three hundred feet. Along these hard and level roads there is always pa.s.sing, during the daytime of the harvesting season, a constant stream of ants--those going from the nest being empty-handed, and those returning to it being laden with seeds. Of course the incoming ants, converging from all quarters upon the road, and therefore increasing in numbers as they approach the nest, require greater s.p.a.ce for free locomotion; while the outgoing ants, diverging as they get further from home, also require greater proportional s.p.a.ce the less their distance from the nest: hence the gradual swelling in the width of the roads as they approach the nests.

The manner of collecting the seeds in the jungle surrounding the roads is thus described by MacCook:--

At last a satisfactory seed is found. It is simply lifted from the ground, or, as often happens, has to be pulled out of the soil into which it has been tightly pressed by the rain or by pa.s.sing feet. Now follows a movement which at first I thought to be a testing of the seed, and which, indeed, may be partially that; but finally I concluded that it was the adjusting of the burden for safe and convenient carriage. The ant pulls at the seed-husk with its mandibles, turning and pinching or 'feeling' it on all sides. If this does not satisfy, and commonly it does not, the body is raised by stiffening out the legs, the abdomen is curved underneath, and the apex applied to the seed. I suppose this to be simply a mechanical action for the better adjusting of the load. Now the worker starts homeward. It has not lost itself in the mazes of the gra.s.s forest. It turns directly towards the road with an unerring judgment. There are many obstacles to overcome. Pebbles, pellets of earth, bits of wood, obtruding rootlets, or bent-down spears of gra.s.s block up or hinder the way. These were scarcely noticed when the ant was empty-handed. But they are troublesome barriers now that she is burdened with a seed quite as thick, twice as wide, and half as long as herself. It is most interesting to see the skill, strength, and rapidity with which the little harvester swings her treasure over or around, or pushes it beneath these obstacles. Now the seed has caught against the herbage as the porter dodges under a too narrow opening. She backs out and tries another pa.s.sage. Now the sharp points of the husk are entangled in the gra.s.s. She jerks or pulls the burden loose, and hurries on. The road is reached, and progress is comparatively easy. Holding the grain in her mandibles well above the surface, she breaks into what I may describe with sufficient accuracy as 'a trot,' and with little further interruption reaches the disk and disappears within the gate. There are variations from this behaviour, more or less marked, according to the nature of the grounds, the seeds, and (I suppose) the individuality of the harvesters; but the mode of ingathering the crop is substantially as above. Each ant operated independently. Once only did I see anything like an effort to extend sympathy and aid. A worker minor seeming to have difficulty in testing or adjusting a large seed of buffalo-gra.s.s, was a.s.sisted (apparently) by one worker major, and then by another, after which she went on her way.

But these ants do not confine their harvesting operations to gathering fallen seeds; they will, like the ants of Europe, also cut seeds from the stalk.

In order to test the disposition of _crudelis_ to garner the seeds from the stem, bunches of millet were obtained from the North, and stalks eighteen inches high, crowned by the boll of close-set seeds, were stuck in the mound of an active formicary. The ants mounted the stems and set to work vigorously to secure the seeds, cl.u.s.ters of twenty or more being engaged at once upon one head. The seeds were carried off and stored within the nest. This experiment proved pretty conclusively that in the seeding season _crudelis_ does not wait for the seeds to drop, but harvests them from the plant.

The 'granaries' into which the seeds are brought are kept distinct from the 'nurseries' for the pupae. Their walls, floor, and roof are so hard and smooth, that MacCook thinks the insects must practise upon them 'some rude mason's craft.'

He traced these granaries to a depth of four feet below the surface of the ground, and believes, from the statements of a native peasant, that they, or at least the formicaries, extend to a depth of fifteen feet.

As regards the care that the ants take of the gathered grain, Lincec.u.m describes the same habit as Moggridge and Sykes describe--viz., the sunning of wet seeds to dry. MacCook, however, neglected to make any experiments on this subject. Neither has he been able to throw any light upon the question as to why the stored seeds do not germinate, and is doubtful whether the habit of gnawing the radicle of sprouting seeds, which prevails in the European species, is likewise practised by the American. On two other points of importance MacCook's observations are also incomplete. One of these has reference to an alleged statement, which he is disposed to believe, that when some ants in a community have been killed by poison, the survivors avoid the poison: he, however, made no experiments to test this statement.

The other main point on which his observations are defective has reference to a remarkable statement made by Lincec.u.m in the most emphatic terms. This statement is that upon the surface of their disk the ants sow the seeds of a certain plant, called ant-rice, for the purpose of subsequently reaping a harvest of the grain. There is no doubt that the ant-disks do very often support this peculiar kind of gra.s.s, and that the ants are particularly fond of its seed; but whether the plant is actually sown in these situations by the insects, or grows there on account of these situations being more open than the general surface of the ground--this question MacCook has failed to answer, or even to further. We are, therefore, still left with Dr. Lincec.u.m's emphatic a.s.surance that he has witnessed the fact. His account is that the seed of the ant-rice, which is a biennial plant, is sown in time for the autumnal rains to bring up. At the beginning of November a green row or ring of ant-rice, about four inches wide, is seen springing up round the circ.u.mference of the disk. In the vicinity of this circular ring the ants do not permit a single spire of any other gra.s.s or weed to remain a day, but leave the aristida, or ant-rice, untouched until it ripens, which occurs in June of the next year. After the maturing and harvesting of the seed, the dry stubble is cut away and removed from the pavement or disk, which is thus left unenc.u.mbered until the ensuing autumn, when the same species of gra.s.s again appears as before, and so on. Lincec.u.m says he has seen the process go on year after year on the same ant-farms, and adds,--

There can be no doubt of the fact that the particular species of grain-bearing gra.s.s mentioned above is intentionally planted. In farmer-like manner the ground upon which it stands is carefully divested of all other gra.s.ses and weeds during the time it is growing. When it is ripe the grain is taken care of, the dry stubble cut away and carried off, the paved area being left unenc.u.mbered until the ensuing autumn, when the same 'ant-rice' reappears within the same circle, and receives the same agricultural attention as was bestowed upon the previous crop--and so on year after year, as I _know_ to be the case, in all situations when the ant's settlements are protected from graminivorous animals.

In a second letter Dr. Lincec.u.m, in reply to an inquiry from Mr. Darwin whether he supposed that the ants plant seeds for the ensuing crop, says:--

I have not the slightest doubt of it. And my conclusions have not been arrived at from hasty or careless observation, nor from seeing the ants do something that looked a little like it, and then guessing at the results. I have at all seasons watched the same ant-cities during the last twelve years, and I know that what I stated in my former letter is true.

I visited the same cities yesterday, and found the crop of ant-rice growing finely, and exhibiting also the signs of high cultivation, and not a blade of any other kind of gra.s.s or weed was to be seen within twelve inches of the circular row of ant-rice.--(_Journ. Linn. Soc._, vol. vi. p. 30-1.)

Now, MacCook found the ant-rice growing as described, but only on some nests. Why it does not grow upon all the nests he does not understand.

So far, then, as his observations go, they confirm those of Dr.

Lincec.u.m; but he does 'not believe that the ants deliberately sow a crop as Lincec.u.m a.s.serts;' he thinks 'that they have for some reason found it to their advantage to permit the aristida to grow upon their disks, while they clear off all other herbage;' but finally concludes 'that there is nothing unreasonable, nor beyond the probable capacity of the emmet intellect, in the supposition that the crop is actually sown.

Simply, it is the Scotch verdict--"Not proven."'

The following facts with regard to 'modes of mining' are worth quoting from MacCook:--

In sinking the galleries the difficulty of carrying is not great in a moist or tough soil, which permits the ant to obtain goodly-sized pellets for portage. But when the soil is light and dry, so that it crumbles into dust as it is bitten off, the difficulty is greatly increased. It would be a very tedious task indeed to take out the diggings grain by grain. This difficulty the worker overcomes by balling the small particles against the surface of the gallery, the under side of the head, or within and against the mandibles. The fore-feet are used for this purpose, being pressed against the side face, turned under, and pushed upward with a motion similar to that of a man putting his hand upon his mouth. The abdomen is then swung underneath the body and the apex pressed against the little heap of grains of dirt ma.s.sed against the under side of the mandibles, or between that and the smooth under surface of the head. Thus the dust is compressed into a ball which is of sufficient size to justify deportation.

The same operation is observed in the side-galleries, where the ants work very frequently upon their sides or backs, precisely as I have seen colliers do in Pennsylvania coal-mines.

The following is likewise worth quoting from the same author:--

Seeds are evidently not the only food of our agriculturals. When the ants at disk No. 2 had broken through the slight mud-sediment that sealed up their gate, as described above, they exhibited a peculiar behaviour. Instead of heading for the roads and pressing along them, they distributed themselves at once over the entire disk, radiating from the gate to all points in the circ.u.mference, from which they penetrated the jungle of gra.s.s beyond. In a moment a large number were returning across the roads, out of the gra.s.s, over the pavement toward the entrance. They bore in their mandibles objects which I presently found to be the males and females of white ants (_Termes flavipes_), which were filling the air, during and after the rain, in marriage flight. They had probably swarmed just before the shower. The agriculturals were under great excitement, and hurried forth and back at the top of their speed. The number of ants bearing termites was soon so great that the vestibule became choked, and a ma.s.s of struggling anthood was piled up around the gate. A stream of eager insects continually poured out of the door, pus.h.i.+ng their way through the crowd that vainly but persistently endeavoured to get in with their burdens.

The outcoming ants had the advantage, and succeeded in jostling through the quivering rosette of antennae, legs, heads, and abdomens. Occasionally a worker gained an entrance by dint of sheer physical force and perseverance. Again and again would the crowd rush from all sides upon the gate, only to be pushed back by the issuing throng. In the meanwhile quite a heap of termites, a good handful at least, had been acc.u.mulated at one side of the gate, the ants having evidently dropped them, in despair of entrance, and hurried off to garner more.

In due time the pressure upon the vestibule diminished, the laden workers entered more freely, and in the end this heap was transferred to the interior.

The rapidity with which the ants were distributed to all parts of their roads, after the first opening of the gates, was truly surprising. I was greatly puzzled, at the first, to know what the cause of such a rush might be. The whole behaviour was such as to carry the conviction that they knew accurately what effect the rain would have, had calculated upon it, and were acting in accordance with previous experience. I had no doubt at the time, and have none now, that the capturing of insects beaten down by the rain is one of the well-established customs of these ants. I saw a few other insects taken in, and one milliped, but chiefly the white ants.

That very afternoon I found in a formicary which I then opened several large colonies, or parts of one colony of termites, nested within the limits of the disk and quite at home. The next day numbers of the winged white ants were found stored within the granaries of a large formicary. There is no reason to doubt that these insects were intended for food, in accordance with the quite universal habit of the _Formicariae_.

A curious habit has been noticed by most observers to occur in many species of ant, and it is one on which Mr. MacCook has a good deal to say. The habit in question consists in the ants transporting one another from place to place. The carrying ant seizes her comrade by the middle, and hurries along with it held aloft--the ant which is carried remaining quite motionless with all her legs drawn together. Huber supposed the process to be one enjoyable to both the insects concerned, and to be performed by mutual understanding and consent; but MacCook, in common with most other observers, supposes that it is merely a rough and primitive way of communicating to fellow-workers the locality where their services are required. He says:--

Keeping these facts in mind, we have a key to the solution of the press-gang operations which Lincec.u.m observed among the agriculturals, and which have been fully described in other species. In the absence of any common head or directory, and of all executive officers, a change of location or any other concerted movement must be carried forward by the willing co-operation of individuals. At first sight, the act of seizing and carrying off workers does not appear like an appeal to free-will. It is indeed coercive, so far as the first act goes. But, in point of fact, the coercion ceases the moment the captive is set down within the precincts of the new movement. The carrier-ant has depended upon securing her consent and co-operation by thus bringing her within the circle of activity for which her service is sought. As a rule, no doubt, the deported ant at once yields to the influence around her, and drops into the current of fresh enterprise, in which she moves with as entire freedom and as independently as any other worker. But she is apparently under no restraint, and if she so please, may return to her former haunts.

_Certain Ants of Africa._--Livingstone says of certain ants of Africa:--

They have established themselves on the plain where water stands so long annually as to allow the lotus and other aqueous plants to come to maturity. When all the ant-horizon is submerged a foot deep, they manage to exist by ascending to little houses built of black tenaceous loam on stalks of gra.s.s, and placed higher than the line of inundation. This must have been the result of experience, for, if they had waited till the water actually invaded their terrestrial habitations, they would not have been able to procure materials for their aerial quarters, unless they dived down to the bottom for every mouthful of clay.[42]

_The Tree Ant of India and New South Wales._--These ants are remarkable from their habit of forming nests only in trees. According to Col.

Sykes' account, the shape of the nest is more or less globular, and about ten inches in diameter. It is formed entirely of cow-dung, which the insects collect from the ground beneath, and work into the form of thin scales. These are then built together in an imbricated manner, like tiles or slates upon the roof of a house, the upper or outer scale, however, being one unbroken sheet, which covers the whole nest like a skull-cap. Below this the scales are placed one upon another in a wavy or scalloped manner, so that numerous little arched entrances are left, and yet, owing to the imbricated manner in which the scales are arranged, the interior of the nest is perfectly protected from rain.

This interior consists of a number of irregular cells, the walls of which are formed by the same process as the exterior.

In New South Wales there is another species of ant which also frequents trees, but builds within the stem and branches. In the report of Captain Cook's expedition its habits are thus described:--'Their habitations are the insides of the branches of a tree, which they contrive to excavate, by working out the pith almost to the extremity of the slenderest twig; the tree at the same time flouris.h.i.+ng as if it had no such inmate.' On breaking one of the branches the ants swarm out in legions. Some of our native species also have the habit of excavating the interior of trees, though not on so extensive a scale.

_Honey-making Ant_ (Myrmecocystus mexica.n.u.s).--This ant is found in Texas and New Mexico. Capt. W. B. Fleeson has observed its habits, and his observations have been communicated to the Californian Academy of Sciences, and also, by Mr. Henry Edwards, to Mr. Darwin. The following are the chief points of interest in Capt. Fleeson's results:--

The community appears to consist of three distinct kinds of ants, probably of two separate genera, whose offices in the general order of the nest would seem to be entirely apart from each other, and who perform the labour allotted to them without the least encroachment upon the duties of their fellows. These three kinds are--

I. Yellow workers; nurses and feeders of II.

II. Yellow honey-makers; sole function to secrete a kind of honey in their large globose abdomens, on which the other ants are supposed to feed. They never quit the nest, and are fed and tended by I.

III. Black workers, guards, and purveyors; surround the nest as guards or sentinels, in a manner presently to be described, and also forage for the food required for I. They are much larger and stronger insects than either I. or II., and are provided with very formidable mandibles.

The nest is placed in sandy soil in the neighbourhood of shrubs and flowers, is a perfect square, and occupies about four or five square feet of ground, the surface of which is kept almost unbroken. But the boundaries of the nest are rendered conspicuous by the guard of black workers (III.), which continuously parade round three of its sides in a close double line of defence, moving in opposite directions. In the accompanying diagram this sentry path is represented by the thick black lines. These always face the same points of the compa.s.s, and the direction in which the sentries march is one column from south-west to south-east, and the other column from south-east to south-west--each column, however, moving in regular order round three sides of a square.

The southern side of the encampment is left unguarded; but if any enemy approaches on this or any other side, a number of the guards leave their stations, and sally forth to face the foe--raising themselves on their hind tarsi on meeting the enemy, and moving their large mandibles in defiance. Spiders, wasps, beetles, and other insects, if they venture too near the nest, are torn to pieces by the guard in a most merciless manner, and the dead body of the vanquished is speedily removed from the neighbourhood of the nest--the guard then marching back to resume their places in the line of defence, their object in destroying other insects being the defence of their encampment, and not the obtaining of food.

The object of leaving the southern side of the square encampment open is as follows. While some of the black workers are engaged on duty as guard, another and larger division are engaged on duty as purveyors.

These enter and leave the quadrangle by its open or southern side along the dotted line marked _a_ to the central point _c_. The incoming line is composed of individuals each bearing a burden of fragments of flowers or aromatic leaves. These are all deposited in the centre of the quadrangle _c_. Along the other diagonal _e_ there is a no less incessantly moving double line of yellow workers (I.), whose office it is to convey the supplies deposited by the black workers at _c_ to _b_, which is the gateway of the fortress. It is remarkable that no black ant is ever seen upon the line _e_, and no yellow one upon the line _a_; each keeps his own separate station, and follows his own particular duty with a steadfastness and apparent adherence to discipline that are most astonis.h.i.+ng. The hole at _d_ seems to be a ventilating shaft; it is never used as a gateway.

[Ill.u.s.tration: Fig. 7.]

Section of the nest reveals, besides galleries, a small chamber about three feet below the surface, across which is spread, like a spider's web, a network of squares spun by the insects, the squares being about 1/4 inch across, and the ends of the whole net being fastened to the earthen walls of the chamber. In each one of the squares, supported by the web, sits one of the honey-making ants (II.). Here these honey makers live in perpetual confinement, and receive a constant supply of flowers, pollen, &c., which is continually being brought them by (I.), and which, by a process a.n.a.logous to that performed by the bee, they convert into honey.

Such is an epitome of the only account that the world has yet received of the habits and economy of this wonderful insect, whose instincts of military organization seem to be not less wonderful than those of the Ecitons, though in this case they are developed with reference to defence, and not to aggression. It is especially noteworthy that the black and yellow workers are believed to belong to 'two separate genera;' for if this is the case, it is the only one I can recall of two distinct species co-operating for a common end; for even the nearest parallel which we find supplied in other species of ants maintaining aphides, is not quite the same thing, seeing that the aphides are merely pa.s.sive agents, like Cla.s.s II., of the honey-making ant, and not actively co-operating members of the community, like Cla.s.s I.

_Ecitons._--We have next to consider the habits of the wonderful 'foraging,' or, as it might be more appropriately called, the military ant of the Amazon. These insects, which belong to several species of the same genus, have been carefully watched by Belt, Bates, and other naturalists. The following facts must therefore be accepted as fully established.

_Eciton legionis_ moves in enormous armies, and everything that these insects do is done with the most perfect instinct of military organization. The army marches in the form of a rather broad and regular column, hundreds of yards in length. The object of the march is the capture and plunder of other insects, &c., for food, and as the well-organised host advances, its devastating legions set all other terrestrial life at defiance. From the main column there are sent out smaller lateral columns, the composing individuals of which play the part of scouts, branching off in various directions, and searching about with the utmost activity for insects, grubs, &c., over every log, under every fallen leaf, and in every nook and cranny where there is any chance of finding prey. When their errand is completed, they return into the main column. If the prey found is sufficiently small for the scouts themselves to manage, it is immediately seized, and carried back to the main column; but if the amount is too large for the scouts to deal with alone, messengers are sent back to the main column, whence there is immediately dispatched a detachment large enough to cope with the requirements. Insects which when killed are too large for single ants to carry, are torn in pieces, and the pieces conveyed back to the main army by different individuals. Many insects in trying to escape run up bushes and shrubs, where they are pursued from branch to branch and twig to twig by their remorseless enemies, until on arriving at some terminal ramification they must either submit to immediate capture by their pursuers, or drop down amid the murderous hosts beneath. As already stated, all the spoils that are taken by the scouts or by the detachments sent out in answer to their demands for a.s.sistance, are immediately taken back to the main column. When they arrive there, they are taken to the rear of that column by two smaller columns of carriers, which are constantly running, one on either side of the main column, with the supplies that are constantly pouring in from both sides. Each of these outside columns is a double line, the ants composing one of the two lines all running in the same direction as the main army, and the ants composing the other line all running in the opposite direction. The former are empty-handed carriers, which having deposited their burdens in the rear, are again advancing to the van for fresh burdens. Those composing the other line are all laden with the mangled remains of insects, pupae of other ants, &c. On either side of the main column there are also constantly running up and down a few individuals of smaller size and lighter colour than the other ants, which seem to play the part of officers; for they never leave their stations, and while running up and down the outsides of the column, they every now and again stop to touch antennae with some member of the rank and file, as if to give instructions. When the scouts discover a wasp's nest in a tree, a strong force is sent out from the main army, the nest is pulled to pieces, and all the larvae carried to the rear of the army, while the wasps fly around defenceless against the invading mult.i.tude. Or, if the nest of any other species of ant is found, a similarly strong force, or perhaps the whole army is deflected towards it, and with the utmost energy the innumerable insects set to work to sink shafts and dig mines till the whole nest is rifled of its contents. In these mining operations the ants work with an extraordinary display of organized co-operation; for those low down in the shafts do not lose time by carrying up the earth which they excavate, but pa.s.s on the pellets to those above; and the ants on the surface, when they receive the pellets, carry them, 'with an appearance of forethought that quite staggered' Mr. Bates, only just far enough to ensure that they shall not roll back again into the shaft, and, after depositing them, immediately hurry back for more. But there is not a rigid division of labour, although the work 'seems to be performed by intelligent co-operation amongst a host of eager little creatures;' for some of them act 'sometimes as carriers of pellets, and at another as miners, and all shortly afterwards a.s.sume the office of conveyors of the spoil.' Again, as showing the instincts of co-operation, the following may also be quoted from Bates's account:--

On the following morning no trace of ants could be found near the place where I had seen them the preceding day, nor were there signs of insects of any description in the thicket; but at the distance of eighty or one hundred yards, I came upon the same army, engaged evidently on a razzia of a similar kind to that of the previous evening; but requiring other resources of their instinct, owing to the nature of the ground. They were eagerly occupied on the face of an inclined bank of light earth in excavating mines, whence, from a depth of eight or ten inches, they were extracting the bodies of a bulky species of ant of the genus Formica. It was curious to see them crowding round the orifices of the mines, some a.s.sisting their comrades to lift out the bodies of the Formicae, and others tearing them in pieces, on account of their weight being too great for a single Eciton; a number of carriers seizing each a fragment, and carrying it off down the slope.

These Ecitons have no fixed nest themselves, but live, as it were, on a perpetual campaign. At night, however, they call a halt and pitch a camp. For this purpose they usually select a piece of broken ground, in the interstices of which they temporarily store their plunder. In the morning the army is again on the march, and before an hour or two has pa.s.sed not a single ant is to be seen where the countless mult.i.tudes had previously covered the ground.

Another and larger species of Eciton (_E. humata_) hunts sometimes in dense armies, and sometimes in columns, according to the kind of prey of which they are in search. When in columns they are seeking for the nests of a certain species of ant which have their young in holes of rotten logs. These Ecitons when seeking for these nests hunt about, like those just described, in columns, which branch off in various directions. When a fallen log is reached, the column spreads over it, searching through all the holes and cracks. Mr. Belt says of them:--

The workers are of various sizes, and the smallest are here of use, for they squeeze themselves into the narrowest holes, and search out their prey in the furthest ramifications of the nests. When a nest of the _Hypoclinea_ is attacked, the ants rush out, carrying the larvae and pupae in their jaws, but are immediately despoiled of them by the Ecitons, which are running about in every direction with great swiftness. Whenever they come across a _Hypoclinea_ carrying a larva or pupa, they take it from it so quickly, that I could never ascertain exactly how it was done.

As soon as an Eciton gets hold of its prey, it rushes off back along the advancing column, which is composed of two sets, one hurrying forward, the other returning laden with their booty, but all and always in the greatest haste and apparent hurry. About the nest which they are harrying, all appears in confusion, Ecitons running here and there and everywhere in the greatest haste and disorder; but the result of all this apparent confusion is that scarcely a single _Hypoclinea_ gets away with a pupa or larva. I never saw the Ecitons injure the Hypoclineas themselves, they were always contented with despoiling them of their young.

The columns of this species 'are composed almost entirely of workers of different sizes;' but, as in the species previously mentioned, 'at intervals of two or three yards there are larger and lighter coloured individuals that often stop, and sometimes run a little backward, stopping and touching some of the ants with their antennae,' and looking 'like officers giving orders and directing the march of the column.'